The invention relates to a vacuum generating means for a vacuum sewer system and inn particular to the use of an ejector as a vacuum pump in such a sewer system.
Ejectors have long been used as a source of partial vacuum in vacuum sewer systems. Such an arrangement is shown in U.S. Pat. No. 4,034,421. According to this known art, the working medium of the ejector is a flow of liquid fed by a circulation pump to the ejector from a sewage collecting container. The total efficiency rate of such a vacuum generating means is only about 5 percent. This is because the efficiency rate of the circulation pump is about 40 percent and only about 10 percent to 15 percent of its useful power can be utilized in the ejector that it drives. An improvement in the efficiency rate of the vacuum generating means is, however, not usually of any great importance per se.
Improving the total efficiency rate of a liquid-driven ejector working as an air pump has been the subject of extensive research. Nevertheless, the aim of the present invention is not to improve the efficiency rate of the ejector. This invention is based on the idea that in the special application of an ejector as the vacuum generating means for a vacuum sewer system it is more important to maximize the air flow drawn into the ejector at a sufficiently high vacuum level (higher vacuum=smaller absolute pressure). In a vacuum generating means according to the invention, the ejector discharges directly into the sewage collecting container (e.g. at atmospheric pressure). Under these circumstances the pressure and the kinetic energy of the mass flow from the ejector are not utilized at all and this has a decisive influence on the optimizing of the function of the ejector.